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Isoniazid (INH)

Prescription

Antimycobacterial (First-Line Anti-TB)

Last reviewed 21 Apr 2026 · PetCare.AI Editorial Team

Species

Dog, Cat

Brands

1 available

Interactions

2 documented

Formulations

Mechanism of action

Prodrug activated by mycobacterial catalase-peroxidase (KatG) to form isonicotinic acyl radical which inhibits InhA (enoyl-ACP reductase), blocking mycolic acid synthesis — essential for mycobacterial cell wall. Bactericidal against actively growing M. tuberculosis.

At a glance

Class

Antimycobacterial (First-Line Anti-TB)

Schedule

Prescription

Storage

Store at room temperature, protect from light and moisture

Dosing

🐕

Dog

Used for M

Dose

10–20 mg/kg

Route

Frequency

q24h

🐈

Cat

Dose

10–15 mg/kg

Route

Frequency

q24h

Educational reference only

This information is provided for educational purposes and is not a substitute for professional veterinary advice, diagnosis, or treatment. Always consult a qualified veterinarian before administering any medication to your pet. Find a vet near you →

Formulations

💊

Other — 2

Strength

—

Strength

—

Storage

Store at room temperature, protect from light and moisture

Safety

Monitoring parameters

Hepatic enzymes q2–4 weeks initially, then monthlyNeurological exam (neuropathy)CBCCulture response

Educational reference only

Interactions

Major — 1

Phenobarbital

major

Isoniazid inhibits CYP enzymes metabolizing phenobarbital, increasing phenobarbital levels and toxicity. Isoniazid is also inherently hepatotoxic — additive liver injury risk.

Management: Monitor phenobarbital levels. Monitor hepatic enzymes closely. May need phenobarbital dose reduction.

Moderate — 1

Ketoconazole (Systemic)

moderate

Isoniazid may reduce ketoconazole absorption. Ketoconazole requires acidic gastric pH for dissolution — isoniazid may alter this.

Management: Separate by 2 hours. Monitor antifungal response.

Educational reference only

Brands

Other markets

Nydrazid

FAQs

Frequently asked questions

›What is Isoniazid (INH)?

Isoniazid (INH) is a antimycobacterial (first-line anti-tb) used in pets. Prodrug activated by mycobacterial catalase-peroxidase (KatG) to form isonicotinic acyl radical which inhibits InhA (enoyl-ACP reductase), blocking mycolic acid synthesis — essential for mycobacterial cell wall. Bactericidal against actively growing M. tuberculosis.

›What is Isoniazid (INH) used for in pets?

Isoniazid (INH) is used in veterinary medicine for: Used for M; M.

›What is the Isoniazid (INH) dose for dogs?

For dogs, Isoniazid (INH) is typically dosed as follows — Used for M: 10–20 mg/kg PO q24h. Always consult your veterinarian for a dose tailored to your pet's weight, age, and condition.

›What is the Isoniazid (INH) dose for cats?

For cats, Isoniazid (INH) is typically dosed as follows — M: 10–15 mg/kg PO q24h. Always consult your veterinarian for a dose tailored to your pet's weight, age, and condition.

›Does Isoniazid (INH) need a prescription?

Yes. Isoniazid (INH) is a prescription medication and should only be administered under veterinary supervision.

Educational reference only

References

The PetCare.AI drug reference is built from 13 authoritative sources cited across 580 drug monographs.

Textbooks & handbooks — 5

Plumb's Veterinary Drug Handbook
Withrow & MacEwen's Small Animal Clinical Oncology
Merck Veterinary Manual
NRC Nutrient Requirements of Dogs and Cats
Veterinary Pharmacology and Therapeutics (Riviere & Papich)

Clinical guidelines & consensus — 4

ASPCA Animal Poison Control Center Guidelines
AAHA Diabetes Management Guidelines
ASPCA Poison Control Guidelines
RECOVER CPR Guidelines

Journals & peer-reviewed studies — 2

EPIC Study (J Vet Intern Med 2016)
JVIM FAT CAT Study

Regulatory & approvals — 1

CDSCO Veterinary Drug Approval Registry (1969–2026)

Databases — 1

Washington State University VCPL MDR1 Database